Many building automation or industrial processes need to sense signals for a variety of process variable values within the controlled processes. Typically the sensed signal is converted by the sensing device into a voltage, current, resistance or other interface signal value and the signal value is typically proportional (linearly, nonlinearly, or other) to the sensed value.
Although there are numerous standard values used for voltage (such as 0-1V, 0-2V, 0-5V, 0-10V, 2-10V) and for current (0-1 mA, 0-2 mA, 0-10 mA, 0-20 mA, 4-20 mA), resistance values vary over a wide spectrum of values. Temperature sensors, such as a thermistor, for example may be 100 Ohm, 1,000 Ohm, 2,000 Ohm, 10,000 Ohm, and even 100,000 Ohm. Temperature sensors typically are specified at a reference resistance at a reference temperature and then supply the affect of changing temperature in equation form associated with resistance. A 100,000 ohm thermistor ranges from 33,000 to over 2,200,000 ohms depending on the sensor temperature. Additionally, many position sensors are basically a potentiometer (“pot” or variable resistance) where sensed motion changes the pot resistance. These sensors are typically 0-100 Ohm, 0-1,000 Ohm, 0-2,000 Ohm, 0-10,000 Ohm, 0-100,000 Ohm, or 0-1,000,000 Ohm but can be virtually any other beginning/ending value. Some sensors are non-linear meaning a fixed change in the sensed value at low and high ends of the sensor does not result in equal changes of resistance values. A design solution may use different input circuitry for each type of sensor range, but may expand the circuitry to allow multiple input types. Hardware, software, and or manually controlled switches may be employed to select/deselect various components or reference values.
When a circuit design uses techniques to allow a wider range of these resistance types, the result is typically a loss in the resolution of the sensed value for any/most/all of the individual ranges. That is, a single circuit for sensing 0-1000 and 0-2000 ohm values may use only half of the full scale range of the analog to digital converter verses the full range for the 0-2000 ohm sensor.
In view of the foregoing, there is an ongoing need for systems, apparatuses and methods for determining the resistance values over a wide resistance range without loss of sensed value resolution.